This invention relates to the shape of the rotor shaft of a trimmer capacitor and specifically to the shape of the rotor shaft of a ceramic trimmer capacitor, and contemplates to provide a trimmer capacitor having stable electrostatic capacitance and improved reliability even during changes in the ambient opposed faces of the rotor electrode and a dielectric layer in known trimmer capacitors.
In the conventional ceramic trimmer capacitor as illustrated in FIGS. 1 and 2, a metallic rotor electrode (13) having an outer diameter of about 5 to 8 mm is brought into pressure contact with a dielectric layer (14) by means of a head (12), having a size of about 2 to 3 mm, of a rotor shaft (11). Hence, in fitting the rotor shaft (11) with mechanical impact such as by caulking, a warping or distortion tends to occur on the face of the metallic rotor electrode (13) if strong mechanical impact pressure is applied. If the mechanical impact strength is small, on the other hand, contact becomes insufficient between the rotor electrode (13) and the ceramic dielectric layer (14), thereby causing an air gap between their opposed faces.
The surface of the dielectric layer is generally subjected to a precision mirror finishing so as to have a surface roughness of not greater than 1.mu. by lapping. If unevenness exceeding 1.mu. is present on the surface, however, an air gap is formed and there may thus be a great fluctuation in the electrostatic capacitance, thereby degrading the ambient reliability characteristics of the capacitor with respect to temperature and humidity.